Message Queues (Deep Workflow)

Queues decouple producers and consumers—and introduce duplicates, ordering surprises, poison messages, and lag. Make delivery semantics and failure handling explicit.

When to Offer This Workflow

Trigger conditions:

• - Choosing Kafka vs RabbitMQ vs SQS vs Pub/Sub
• Consumer lag, DLQ growth, reprocessing needs
• “Exactly-once” expectations—need alignment on reality

Initial offer:

Use six stages: (1) delivery semantics, (2) topology & partitions, (3) message contract, (4) consumers & retries, (5) ops & scaling, (6) failure drills). Confirm cloud and ordering requirements.

Stage 1: Delivery Semantics

Goal: Choose at-most-once, at-least-once, or effective-once via idempotency.

Questions

1. 1. Can duplicate processing break invariants?
2. Is ordering global, per-entity, or unnecessary?
3. Latency vs durability trade-offs

Exit condition: One paragraph per pipeline stating semantics.

Stage 2: Topology & Partitions

Goal: Throughput and ordering align—ordering only within partition when using Kafka-style partitions.

Practices

• - Partition key often equals business key (e.g., user id)
• Watch hot partitions

Stage 3: Message Contract

Goal: Versioned events or commands with schema registry.

Practices

• - Envelope: id, type, version, timestamp
• Payload size limits; reference blobs by id

Stage 4: Consumers & Retries

Goal: Exponential backoff + jitter; DLQ with reason; replay tooling owned.

Pitfalls

• - Retries can reorder unless single-threaded per partition

Stage 5: Ops & Scaling

Goal: Lag metrics, consumer offset health, rebalance awareness (Kafka).

Stage 6: Failure Drills

Goal: Kill consumer mid-batch; duplicate publish intentionally; validate idempotency.

Final Review Checklist

• - [ ] Delivery semantics and idempotency explicit
• [ ] Partitioning/ordering strategy documented
• [ ] Versioned message contract
• [ ] Retry, DLQ, replay documented
• [ ] Lag metrics and alerts; capacity plan

Tips for Effective Guidance

• - Exactly-once end-to-end is rare—design for at-least-once + idempotent handlers.
• Challenge global ordering requirements—they cost scale.
• Visibility timeouts (SQS) differ by product—read the vendor docs.

Handling Deviations

• - Transactional outbox when you need DB + queue consistency without dual writes.